Police arrested senior leaders of Tanzania's main opposition party CHADEMA and hundreds of its supporters ahead of a meeting in the southwest of the country, a party spokesperson said on Monday.

Tundu Lissu, CHADEMA's vice chairman, and the party's Secretary General John Mnyika were among those arrested on Sunday evening, party spokesperson John Mrema told Reuters.

Lissu, a former presidential candidate, has harboured ambitions of contesting in next year's presidential election.

Since coming to power in March 2021 following the death of her predecessor, Tanzania's President Samia Suluhu Hassan has lifted a ban on political rallies and relaxed restrictions on media.

However, she has faced criticism from opposition parties and rights groups for arrests last year of those planning protests against a port management deal.

The latest arrests came hours after police banned a conference planned by CHADEMA's youth wing BAVICHA, saying the event was likely to breach the peace.

Police said the party's youth leaders planned to influence young people across the country to meet and demonstrate in Mbeya, a city in southwest Tanzania, which the party spokesperson denied.

Mrema said the conference was to celebrate International Youth Day, where top party officials, including Chairman Freeman Mbowe, were set to address the youth.

"There are more than 400 people who have been arrested by police," the spokesperson added.

Police spokesperson David Misime did not respond to Reuters requests for comment.

CHADEMA Chairman Mbowe said the party condemned the arrest and was monitoring the situation.

"We demand the immediate and unconditional release of all our leaders, members and loved ones who were arrested in various parts of the country," he said on social media platform X.